1.Field of the Invention
The present invention relates to an image blur prevention device for preventing image blur caused by hand fluctuation or the like in a camera, an optical device, or the like.
2. Related Background Art
As shown in FIG. 19, according to a conventional image blur prevention device of this type, in a camera body 600, sensors 601 and 602 are arranged to detect rotational fluctuations around two perpendicular axes, i.e., Y- and P-axes, parallel to the film surface of the camera. A correction optical system is driven by predetermined drive means 603 and 604 on the basis of outputs from these two sensors so as to reduce an image fluctuation (caused by hand fluctuation of the photographer) caused on an imaging plane 606.
In general, the hand fluctuation frequency of a photographer is about 20 Hz at most. An existing sensor (e.g., a vibration gyroscope) and correction optical system (e.g., a shift optical system to be described later) can satisfactorily follow such a frequency.
In a camera or the like having a focal-plane photographic shutter, however, a frequency of about 100 Hz (this frequency is determined by the curtain travel of the shutter, i.e., a fluctuation period is determined by the start and finish of shutter curtain running) is generated by forward and rearward shutter curtain running. That is, an existing sensor and correction optical system cannot satisfactorily follow such a frequency.
FIGS. 20A to 20E show fluctuation waveforms generated upon driving of a focal-plane photographic shutter generally used in a single-lens reflex camera.
As shown in FIG. 20A, first of all, driving of the forward shutter curtain is started, and the camera is moved in the opposite direction to the moving direction of the forward shutter curtain (the shutter generally runs in a vertical direction) owing to the action/reaction relationship. As shown in FIG. 20C, a downward fluctuation appears. When a time t has elapsed since the start of forward shutter curtain running, the forward shutter curtain running is finished, and the movement of the shutter curtain is stopped. As a result, the camera is moved in the reverse direction, and an upward fluctuation appears, as shown in FIG. 20C.
Since the running time of this shutter curtain is generally several msec, a fluctuation frequency generated by this shutter curtain running is several 10 to several 100 Hz. In general, a fluctuation sensor used to detect a hand fluctuation cannot accurately detect a frequency near 100 Hz. For this reason, as indicated by the solid line in FIG. 20D, the peak of a sensor output waveform delays with respect to the peak of an original fluctuation waveform by a time ts. In addition, a correction system driven to actually correct a fluctuation on an imaging plane on the basis of an output from the above fluctuation sensor exhibits a correction band of about 10 to 100 Hz at most. For this reason, with regard to a fluctuation signal based on a sensor output near 100 Hz, the peak of a correction-system output waveform delays with respect to the peak of a sensor output waveform by a time tc, as shown in FIG. 20D.
As described above, with respect to a fluctuation signal of about 100 Hz, an actual correction operation is executed with a considerable delay. For this reason, as indicated by the actual fluctuation waveform on the imaging plane in FIG. 20E, the fluctuation is not decreased but is increased by this delayed correction. FIG. 20B shows how the rearward shutter curtain is driven to run with a delay corresponding to a shutter time set with respect to the start of driving of the forward shutter curtain. As in the case of the forward shutter curtain, it takes several msec to complete rearward shutter curtain running from the start of running to the finish of running. The resultant fluctuation waveform is therefore represented by a signal of several 100 Hz, and a residual of fluctuation correction similar to the one described above occurs. However, this rearward shutter curtain fluctuation does not adversely affect actual exposure processing on the film surface, the influence of the rearward shutter curtain fluctuation is smaller than that of the forward shutter curtain fluctuation.
If a fluctuation sensor and a correction system in an interchangeable lens are operated in accordance with a start communication signal from a camera body, general correction of an image blur caused by a hand fluctuation of a photographer can be realized. However, satisfactory image blur correction cannot be performed with respect to a high-frequency fluctuation signal generated upon movement of a mechanical member such as a shutter. Any photographic improvement effect cannot be obtained especially in a case wherein a photographing operation is performed at a set shutter speed, e.g., about 1/60 to 1/250 sec, although it depends on the curtain travel (several msec in general) of a shutter used in the camera.